This invention relates to an electrical hydraulic servo actuator, and more particularly to a dynamic characteristic compensating device therefor. Still more particularly, the invention relates to such a compensation device utilizing a pressure feedback technique.
It is desirable that a servo mechanism respond with a high degree of accuracy to a large change in a controlled value, but that it not respond to disturbances in the controlled value. Heretofore, electrical hydraulic servo actuators have been provided with a dynamic characteristic compensating device for the purpose of stabilizing the system against disturbances. The dynamic characteristic compensating device operates according to a so-called "pressure feedback technique" in which the variation in pressure of the hydraulic operating chamber of the actuator is detected and differentiated, and the differentiated signal is then employed as a feedback signal for the input signal, so that the variation in pressure is cancelled out.
Such dynamic characteristic compensating devices can be grouped into two classes. Devices of the first class are purely hydraulically operated. More specifically, in the device of the first class, as shown in FIG. 3, an actuator 100 defines a hydraulic operating chamber 103, and an orifice 102 is provided in a passageway between the actuator 100 and a cushion chamber 101 to hydraulically convert the variation in pressure of the hydraulic operating chamber 103 into a differential hydraulic signal. The differential hydraulic signal thus formed is utilized to operate a leakage valve 104 or a feedback link (not shown).
Compensating devices of the second class are electrically operated. More specifically, as shown in FIG. 4, the pressure in a hydraulic operating chamber 201 defined by an actuator 200 is converted into an electrical signal by means of strain gauges 202, and the electrical signal thus obtained is fed back to an electrical input signal section (a combination circuit 206 which may be an adder or a subtractor depending on the polarities of its input signals) through a demodulator 203, a filter 204 and a differentiator 205.
However, the purely hydraulic compensating device is disadvantageous in the following aspects. A first disadvantage is attributable to the orifice 102. More particularly, when air, which is unavoidably mixed in the operating oil, is pooled at the orifice 102, an extremely large error signal results, thus making the system unstable. Further, since the device requires a leakage valve 104 or a pressure feedback plunger, the device is necessarily intricate in construction, and the leakage valve or pressure feedback plunger is liable to become inoperable directly or indirectly due to foregin matter mixed in the operating oil. Thus, the conventional hydraulically-operated dynamic characteristic compensating device is low in reliability and intricate in construction.
The electrically-operated dynamic characteristic compensating device is also disadvantageous in the following aspects. First, since the output signal of the strain gauge 202 is weak, the device is liable to be affected by noise, i.e., it is low in reliability. Furthermore, the device is also intricate in construction because it is necessary to provide a power source for driving the auxiliary electronic units such as the demodulator 203.